Ferrochromium production



Nov. 24, 1964 w. CHYNOWETH 3,158,464

FERROCHROMIUM PRODUCTION Filed May 25, 1963 High-Chrome Slag Chrome Ore Coke SMELTING FURNACE S|||ca Discard Slag High-Si, Low-S,Fe-Cr Alloy Chrome Ore Q2 TREATING LADLE High -Chrome Slag (20-30 Cr) Low-Si,Low-S,Fe-Cr Alloy INVENTOR. WILLIAM CHYNOWETH BY Q a //e L ATTORNEY United States Patent York Filed May 23, 963, Ser. No. 282,6ll'7 2 Claims. (Cl. id-51) The present invention relates to a lime-free process for the production of ferrochromium alloys substantially free from sulfur impurities.

According to their carbon content, ferrochromium alloys are, generally, subdivided into high-, mediumand low-carbon ferrochrominm alloys. Thus, an alloy containing more than about 4.5 percent by weight of carbon may be defined as a high-carbon alloy while one having less than about 2.0 percent by weight of carbon may be called a low-carbon alloy.

One conventional process presently practiced for the production of high-carbon fenochromium involves the direct reduction of chromium ore with carbon, usually in the form of coke. Ferrochromium produced in this manner, however, is invariably contaminated with sulfur. The presence of sulfur, derived from the coke in the highcarbon terrochromium alloy, is particularly objectionable in steels to which ferrochromium is conventionally added as an alloying agent. Furthermore, a simple carbon-reduction of chromium ore does not provide a melt having the desired fluidity, thus resulting in poor metallurgical operation.

One other known process for producing high-carbon ferrochromium comprises the smelting of chromium ore with carbon and silica. The addition of the silica gives the slag ensuing from this process the recpiired fluidity for easier handling of the melt. However, the carbon content must be controlled by careful selection of ores and close control of slag composition. There are many ores which are not metallurgically useful because of friability and chemical composition. Frrthermore, bulky, highlime slags must often be used in order to produce a low sulfur alloy.

The carbon control may be successfully attained by practicing still another commercially well-known process. This may involve either the carbon reduction of a chromium ore in the presence of silica and of a carbon-controlling fiuxing agent such as lime and the like, or for lowcarbon alloy it may involve the silicon reduction of chromium ore in the presnece of lime, iluorspar and similar fluxing agents, the silicon employed being in the form of elemental silicon, fe-rrosilicon alloys or a ferrochromiumsilicon alloy.

In processes of this type how ever, in order to achieve maximum reduction of chromium as well as to produce a low silicon alloy, heretofore it was necessary to employ a material such as lime in amounts sufficient to obtain in the slag a ratio of between 1.5 and 2.0 parts by weight of lime per part by weight of silica. Consequently, these processes utilizes large qualtities of lime or equivalent fiuxing agents in the silicon oxidizing reaction with a resulting production or" large volumes of waste slag.

One known disadvantage of excessive Waste slag is the inevitable entrapment of metallic chromium therein, usually in the order of 5 percent or more of the slag Weight. This loss places a decisive economic penalty on the overall process. Another known disadvantage of excesisve slag is the waste of sizable quantities of lime or other fluxing materials, and the necessity of handling superfluous slag in quantities larger than desired.

It is accordingly an object of the present invention to provide a lime-free process for the production of ferrochromium alloys having a low silicon content, substantially no sulfur, and a carbon content of from about 0.5 percent by Weight to about 5 percent by weight.

It is another object of this invention to provide a. limefree process for the concurrent production of a ferrochromium alloy and a chromium-enriched slag which may be recycled for further employment in the process.

A further object of the invention is to provide a process amenable to a wider selection of ores than now considered useful with present processes.

it is another object of the invention to provide a relatively low temperature process for the production of ferrochrome alloys having a low silicon content and substantially free of sulfur.

Other objects and advantages of the invention will be apparent from the following detailed description and from the appended claims.

The accompanying clrawin is a flow sheet of a process for the production of low-silicon, low-sulfur ferrochromium alloy.

Briefly stated, the process which satisfies the objects of the invention consists of a two-step metallurgical operation and comprises, in the first step, smelting oxidic chromium ore with carbon and silica to produce a molten high-silicon, low-sulfur, high-carbon intermediate ferrochromium alloy and a low-volume discard slag; and, in the second step, tapping the molten intermediate alloy into a. ladle at a temperature of G C. to 1600" C. and treating the molten alloy in the ladle with fresh oxi-dic chromium ore and gaseous oxygen to oxidize the silicon in the intermediate alloy to silica to produce a low-silicon, low-sulfur alloy, and to provide a high-chromium slag suitable for recycling as a furnace additive to produce additional high-silicon, low-sulfur intermediate ferrochromium alloy.

With reference to the single figure, the oxidic chromiurn ore employed in the smelting step may be any suitable chrome ore, or a mixture of different chrome ores, or a mixture of chrome ores and a recyclable chromium-rich slag such as that obtained in the second step of the process of the invention. The carbonaceous materials, more over, need not be a sulfur--free carbon, because it has been found that the high-silicon intermediate alloy rejects sulfur; thus, any siutable carbonaceous matter may be employed.

The high-silicon content of the intermediate ferrochromiurn alloy produced in the first step of the process is mainly responsible for the low-sulfur content in the final ferrochromium alloy.

In the ladle treating step of the process, any desired portion of the silicon and of the carbon contained in the intermediate alloy are oxidized to silica and carbon monoxide, respectivel It is important however, that the temperature of the molten metal introduced into the ladle be at a temperature of between 1500 C. and 1660" C. to avoid substantial, undesirable metal loss and refractory loss during the ladle treatment. For the same reason, the ladle oxygen treatment is conducted so that the temperature of the molten metal does not rise above about 1760 C. This can be accomplished by controlling the rate of oxygen and chrome ore addition and also by the addition of scrap to the ladle. When the foregoing process temperatures are used and when fresh oxidic chromium ore and gaseous oxygen are introduced in the treating ladle of the second step, silicon is removed preferentially to the carbon and there is practically no loss of chromium. If a high-carbon, low-silicon alloy is desired this may be readily accomplished by introducing a larger quantity of oxidic chromium ore in the ladle, or by continuing the blowing with oxygen until all the undesired silicon has been removed as silica. If, in addition to a low-silicon content in the alloy, a low carbon content is also desired, this can be accomplished by continued addition of chromium ore to the ladle and blowing oxygen, so that after all of the silicon has been oxidized to silica, the carbon, in turn, will be oxidized to carbon monoxide. This procedure is followed also when a highsilicon and low-carbon ferrochromium alloy is desired; in this case, after having removed all of the silicon and the desired amount of carbon, as silica and carbon monoxide respectively, silicon must be reintroduced in quantity sufiicient to give the desired silicon level in the alloy.

In addition to the previously mentioned benefit, the oxidic chromium ore added in the second or ladle treating step of the process fluxes the silica formed in the oxidation of the silicon in the intermediate alloy. A further reaction also takes place in the ladle, namely the silicon of the intermediate alloy reduces the iron contained in the added chromium ore. The iron, by entering the metallic regulus, causes a redistribution of chromium and iron in the slag. A slag is thus produced which has a higher chromium-to-iron ratio than that in the chromium ore employed in the first smelting operation, such ratio being at least 4, whereas most chrome ores have chromium-to-iron ratios of from 1.5 to less then 4.0. This high ratio in the recyclable slag is obtained regardless of the type of chromium ore originally used in the smelting operation.

In the known processes for producing ferrochromium alloys, particularly those alloys having a carbon content of between 0.5 and 4 percent by weight, by the silicon reduction of a chromium ore in the presence of lime as fluxing agent, furnaces are employed which are generally lined with magnesia refractory materials. The instant process, conversely, preferably utilizes chromium ore as a refractory in the ladle treating operation of the process. When a chromium-ore lined ladle is employed, the resultant recyclable slag is not further contaminated by any of the refractory material that may be consumed during the treating operation. This procedure has also a definite economic advantage in that such waste and consumption of refractory linings, occurring heretofore, is practically obviated, thus minimizing refractory costs, improving chromium recovery, and minimizing slag contamination.

The rate at which the oxygen is introduced in the ladle and the length of the treatment may, as stated above, be so controlled that an alloy may be obtained having any desired silicon content and, at the same time, a specified carbon content ranging from about 0.5 to about percent by weight.

The following example is illustrative of the production of a high-carbon, low-silicon, low-sulfur ferrochromium alloy in accordance with the present invention. The procedure followed is that schematically shown in the accompanying drawing. The steps of the process involve smelting a mixture of chromite ores and a high-chromium recycle slag obtained from the second step of the process of a previous run with a mixture of carbonaceous reducing agents and quartzite to produce a molten intermediate low-sufur ferrochromium alloy and a discar slag at tapping temperatures of about 1600 C., and in a second step, a ladle treatment of this molten intermediate alloy with gaseous oxygen and fresh chromium ore,

commencing when the temperature of the molten metal is about 1 00 (3., to produce a low-silicon, low-sulfur, high-carbon ferrochromium alloy and a high-chromium recyclable slag.

The raw materials used in the first step of the example had the following composition:

Components Chromium Chromium Chromium Recycle Ore #1 Ore #2 Ore #3 Slag Fixed Carbon in Carbo- Percent Becker Gravel Percent neceous Materials Pea Coke 86. 69 SiO2 99. 27 Pocohantas 002 1.. 76. 25 0. 10 Wood chips 11.0 3 0. 10 Loss on Ignition 0.41

The raw materials were subsequently mixed in several batches having the following typical weight composition.

Typical batch mix weight com-position: Lb. Chromium ore #1 330 Chromium ore #2 Chromium ore #3 330 Recycle slag Pea coke Pocohantas coal 30 Wood chips 150 Becker gravel 125 These batches were introduced into a three-phase electric arc furnace and the smelting operation was carried out under a submerged are condition. The metal and slag were then tapped at about 1600 C. into a treating ladle lined with chromium ore. A rough separation of the molten intermediate alloy from the discard slag was made by decanting the furnace slag into a separate metallurgical vessel. The average composition of 54 taps of metal and respective slag prior to the second step treatment, made during the run, given in weight percent was as follows:

Intermediate Alloy Percent Discard Slag Percent Chromium 66.67 1 Total Chromium 4.35 22.61 Chromium as Cr O 1.03 3.47 SiOz 34.94 6.25 A1203 23.86 Sulfur 0.027 CaO-l-MgO 32. 20

The total chromium accounted for in this step of the operation was 99.0%

in the second step of the example, 7500 pounds of the tapped, molten intermediate alloy, at a temperature of about 1500 C., were treated with chrome ore and gaseous oxygen in a chromium-ore-lined ladle equipped with a hood and an ore and oxygen feeding apparatus. The ladle hood was lined with an alumina-silica refractory, while the ladle proper was lined with chrome ore. About 125 pounds of chrome ore #1 and 9,750 cubic feet of oxygen were used in the treatment. The oxygen was added to the molten intermediate alloy through a lance-type device positioned centrally in the ladle hood, and extending beneath the surface of the melt. The lance was also lined with an alumina-silica refractory. The oxygen was fed to the intermediate grade alloy undergoing treatment at a rate of 350 cubic feet per minute, so that the total time expended for the oxidizing reaction was about 28 minutes. The maximum temperature of the metal during the oxy gen treatment did not exceed 1760 C. The composition r-ei of the final alloy and of the by-product recyclable slag after treatment was as follows:

Final Alloy I Percent Recyclable Slag Percent Chromium 67. 98 OM0 .t 24. 05 Ir 0.6

The total chromium accounted for in this step of the operation was 97.6%.

The above-given example is illustrative of a process for producing high-carbon, low-silicon ferrochrornium alloys. However, if both the carbon and the silicon levels in the final alloy are desired to be low, this may be attained by prolonging the blowing of the oxygen beyond the 28 minutes given in the example, preferably without the addition of chromium ore.

A similar operation would be effected if a high-silicon and low-carbon ferrochromium alloy were desired. In this case, after having removed the silicon and the required amount of carbon, by means of longer oxygen blowing, the necessary silicon content must be reintroduced by a further addition of silicon.

The present application is a continuationin-part of application Serial No. 59,397, filed September 29, 1960.

What ,is claimed is:

l. A process for the production of ferrochromium alloys which comprises smelting oxidic chrome ore with carbonaceous material and silica, in the absence of additional lime, in a metallurgical furnace, to produce a molten high-silicon, low sulfur intermediate ferrochromiurn alloy and a discard slag; tapping said molten intermediate ferrochrorne alloy into a ladle at a tapping temperature of between 1500 C. and 1600 C.; and contacting the tapped molten metal with oxidic chromium ore and treating the tapped molten metal with gaseous oxygen at a rate such that the temperature of the metal does not exceed 1760 C. to produce a low-silicon, low-sulfur, ferrochromium alloy and a re-cyclable chromium-rich slag.

2. The process in accordance with claim 1, wherein the chromium-rich slag is recycled to the metallurgical furnace for re-employment therein.

References Cited in the file of this patent FOREIGN PATENTS 159,568 Great Britain Feb. 24, 1921 

1. A PROCESS FOR THE PRODUCTION OF FERROCHROMIUM ALLOYS WHICH COMPRISES SMELTING OXIDIC CHROME ORE WITH CARBONACEOUS MATERIAL AND SILICA, IN THE ABSENCE OF ADDITIONAL LIME, IN A METALLURGICAL FURNACE, TO PRODUCE A MOLTEN HIGH-SILICON, LOW SULFUR INTERMEDIATE FERRCHROMIUM ALLOY AND A DISCARD SLAG; TAPPING SAID MOLTEN INTERMEDIATE FERROCHROME ALLOY INTO A LADLE AT A TAPPING TEMPERATURE OF BETWEEN 1500*C. AND 1600*C.; AND CONTACTING THE TAPPED MOLTEN METAL WITH OXIDIC CHROMIUM ORE AND TREATING THE TAPPED MOLTEN METAL WITH GASEOUS OXYGEN AT A RATE SUCH THAT THE TEMPERATURE OF THE METAL DOES NOT EXCEED 1760*C. TO PRODUCE A LOW-SILICON, LOW-SULFUR, FERROCHROMIUM ALLOY AND A RE-CYCLABLE CHROMIUM-RICH SLAG. 